Using somatic cell hybridization techniques we ahve been able to establish the chromosome location of the human immunoglobulin heavy chain genes. At present we are attempting to establish the chromosome location of the human genes for mu and lambda immunoglobulin chains. We have also continued our studies on mouse x human somatic cell hybrids and found that mouse more than human hybrids contain only mouse mitochondrial DNA and human more than mouse hybrids contain only human mitochondrial DNA.